Bring a large pot of lightly salted water to a boil. Add pasta and cook according to directions. Melt butter in a large heavy pan over medium high-heat. Sauté shrimp for 1 minute on each side. Stir in garlic, and cook for 1 minute. Remove shrimp and set aside. Stir in flour and Cajun seasoning. Cook, stirring for 5 minutes. Gradually whisk in milk cook until thickened. Remove from heat, and season with salt and lemon juice. Return shrimp to sauce, and spoon over cooked pasta.